This invention relates to an apparatus for controlling a power converter which converts a DC voltage fed from a DC voltage dividing capacitors that are connected in series into an AC phase voltage having positive, zero and negative three potentials.
When a load such as an induction motor is to be driven by a pulse-width modulation invertor, it is desired that the AC output voltage of the invertor contains harmonic components as little as possible.
To satisfy this requirement, there has been proposed an invertor called a three-level invertor.
Such an invertor has been proposed in, for example, "A Novel Approach to the Generation and Optimization of Three-Level PWM Wave Forms" (PESC '88 Record, April 1988), pp. 1255-1262 (hereinafter referred to as literature 1). This literature 1 proposes a dipolar modulation system which alternatingly outputs positive and negative pulse voltages via zero voltage as a modulation system suited for improving the wave forms and for controlling very small voltages of the three-level invertor. voltage utilization factor decreases since very small voltages are controlled by using pulses having a polarity opposite to the fundamental wave of the output phase voltage. This literature 1 further describes a control system which features a high voltage utilization factor, i.e., describes the shifting toward a unipolar modulation system which outputs a plurality of pulse voltages having only the same polarity as the fundamental wave of the output phase voltage.
On the other hand, problems inherent in the three-level invertor will be represented by such phenomena as imbalance in the capacitances of the capacitors that are connected in series to divide a DC voltage into two, and imbalance in the DC components between the divided two DC voltages caused by DC components of a current flowing into and out of a point at which the capacitors are connected in series due to variance in the output pulses from the invertor. Technologies which suppress this imbalance have been disclosed in Japanese Patent Laid-Open No. 101969/1990 and in "Balancing of DC Input Capacitor Voltages of an NPC Invertor" (Material of the Society of Study, Japanese Association of Electric Engineering, Society for Studying Semiconductor Power Conversion, SPC-91-37, 1991/6), pp. 111-120 (hereinafter referred to as literature 2).
According to the technology for suppressing imbalance of DC components between the two DC voltages disclosed in Japanese Patent Laid-Open No. 101969/1990, the amplitudes of two modulated waves of sinusoidal shapes are changed in the dipolar modulation system of the literature 1 . As will be described later, however, this technology causes the shifting amount (bias amount) to be changed, too. To bring the change back, the shifting amount must be adjusted again in the latter stage of control, causing the control operation to become complex.
According to the technology of suppressing the imbalance of DC components between the two DC voltages disclosed in the literature 2, a signal corresponding to a DC component of a differential voltage between the two DC voltages is superposed on an invertor voltage instruction in the unipolar modulation system of the literature 1.
Now, when the above-mentioned three-level invertor is adapted to an electric car, the output voltage is continuously controlled from zero through up to near a possible maximum voltage. Therefore, it becomes necessary to shift the modulation system (modulation mode) from the dipolar modulation system (modulation mode) into the unipolar modulation system (modulation mode).
Here, however, the circuit constitution and control operation become complex if the above-mentioned technologies for suppressing the imbalance of DC components between the two DC voltages are adapted depending upon the modulation systems.